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Good Librations

StarDate:

March 21, 2016

As the Moon orbits Earth, the same hemisphere always faces our way. But it’s like the face of a bobblehead doll — it bounces around a bit. Over the course of the Moon’s month-long cycle of phases, this “bounciness” allows us to see a total of 59 percent of the lunar surface. And until spacecraft began flying around the Moon, that provided the only way to see any of the Moon’s hidden farside.

Several factors contribute to these motions, which are known as librations. The main ones are the Moon’s tilt on its axis and its changing orbital speed.

The Moon’s axis is tilted a bit with respect to its orbit around Earth. That causes first its north pole and then its south pole to tip toward us. That allows us to see over the poles, revealing a smidgen of the Moon’s farside.

And the Moon’s distance from Earth changes by about 30,000 miles, which causes the Moon to speed up and slow down as it goes around our planet. But the Moon spins on its axis at a near-constant speed. The difference in those two motions allows us to see around the Moon’s eastern and western limbs — adding tiny slivers of lunar territory to the view from Earth.

And the view is especially impressive tonight, as the almost-full Moon keeps company with what looks like a brilliant star: Jupiter. The giant planet is quite close to the upper left of the Moon as darkness falls, and stays close to our satellite world as they arc across the southern sky during the night.

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